The present invention relates to an actuator for a rotatable or linearly movable component with at least one defined stop position, and also to an actuator of a braking component of a yarn brake.
An actuator for a yarn brake of a disk brake type known from EP-A-05 97 239 is structured as a permanent magnet motor carrying out a 360xc2x0-rotational movement of a component actuating the yarn within four extremely quick steps. The respective stop position after a step has been carried out is adjusted purely by magnet forces and such that a respective braking position or release position for the yarn is defined. The armature is electromagnetically arrested by the magnet field generated by the excited coil. Due to inertia, the armature tends to continue its rotation beyond an equilibrium position where it is parallel to the respective exciting magnet field. The angle opening between both magnet fields then causes a return torque for the armature. Said return torque defines a quasi-electromagnetic stop. The return torque generated to define the stop position leads to a disadvantageous backwards jerk of the component of the yarn brake which may influence the braking or releasing behaviour of the yarn brake in undesired fashion, i.e., any transition to the respective braking position or releasing position cannot be controlled properly.
Furthermore, controlled yarn brakes having an electric turning actuator are known in practice. Said yarn brakes include a resilient rubber stop defining a stop position. The component of the yarn brake hits the stop after the quick adjustment movement. The result is a jerking motion backwards which may lead to the undesirable effect that the component undesirably influences the yarn even in the braking position or the releasing position, respectively. A jerking movement in the backward direction also may occur at a stopper of a stopping device which is actuated by a linear magnet between a blocking position and a releasing position. Such stop devices are known in yarn feeding devices for jet weaving machines. In this case, the backward jumping motion may allow the weft yarn to slip through, or the withdrawn weft yarn is caught at the stopper.
It is an object of the invention to provide a quick and compact actuator of the kind as disclosed as well as a controlled yarn brake including such an actuator, wherein despite quick adjustment movements the moveable component achieves the respective stop position without jumping back.
Said object can be achieved by providing an additional body at the stop position, the additional body having substantially the mass or the moment of inertia of the drive element and the rotatable or linearly movable component, the additional body being displaceably supported at a returnable motion damping device.
The mass (in case of a linear movement) or the moment of inertia (in case of a rotational movement) of the additional body is precisely matched to the mass or the moment of inertia of the moveable parts. The additional body takes over the entire impact energy without inducing a backward motion. At the stop position, the moveable parts abruptly are brought to a stand still. The additional body continues to move such that its energy will be dissipated in the motion damping device before the additional body returns in delayed fashion into its home position. Said additional body reaches its home position without displacing the parts in the opposition direction which parts already were brought into a stand still condition without jumping back. This results in a yarn brake having the advantage that the moveable parts do not undergo any further swinging motion during the transition from one braking position to another or to a releasing position, and that the intended braking or releasing effect is not influenced detrimentally. Within the time period usually existing for the actuation of controlled yarn brakes, e.g. about 5 ms, the moveable parts are brought to a stand still at a precisely determined position and without jumping back. This is particularly expedient for actuators or yarn brakes, respectively, the working operation of which takes place with a rotation or a linear movement. The actuator may comprise a drive element which is actuated magnetically, electromagnetically, electrically, hydraulically, mechanically or pneumatically. The expedient function results from the inventive measure to first introduce the impact energy totally into the additional body to achieve an absolute stand still and a correct positioning of the moveable parts, and to dissipate the impact energy and to then return the additional body with a delay and damped into the home position such that even then no backwards jump will be generated.
Expediently, the motion damping device for the additional body includes resilient friction damping means and a damped return function with a precisely limited stroke. The additional body brings the moveable part in its home position abruptly to a stand still and then has a longer time to let its energy dissipate and to return into its home position. The time period available for this function corresponds at maximum to the time period between two subsequent working cycles of the actuator in the same direction of movement. Employing the additional body results in a time buffer for energy dissipation and offers the possibility of carrying out the energy dissipation in a precisely predetermined fashion and relatively slowly. During the impact a part of the kinetic energy is converted into heat energy while the remaining part of the kinetic energy is transmitted from the additional body into the motion damping device and is converted into heat energy there. The return function brings the additional body exactly back into the home position, preferably relatively slowly and without causing a jerking motion in a backward direction of the earlier stopped parts.
The additional body can be manufactured simply from a hard or non-resilient material, e.g. from a plastic material like polyurethane. The motion damping device with its return components can be made from a highly resilient material like soft elastic plastic material, e.g. in the form of a polyurethane-foam cushion, which supports the additional body or provides the energy dissipation and return function to return the additional body with a delay to avoid a backward jump of the moveable parts.
It is advantageous to provide two stop devices limiting the working stroke of the actuator or the component, respectively. In each stop device at least one additional body is used to stop the moveable parts. Each stop device, furthermore, can be separated into at least two symmetrical halves which are symmetrical with respect to the axis of the movement of the component such that for a single stop position two additional bodies and two motion damping devices are provided. The mass (for a linear movement of the actuator) or the moment of inertia (for a rotational movement of the actuator) of the additional body should correspond relatively precisely to the mass or the moment of inertia, respectively, of all parts to be moved into the stop position. When separating a stop device into at least two symmetrical halves, of course, the respective additional body only needs to have half of the mass or of the moment of inertia transmitted by the stop element.
An expedient embodiment employs an electric actuator including a linear electromotor or a rotating electromotor. Particularly advantageous is a permanent magnet motor constituting said linear motor or said rotational electromotor. It can be manufactured with few structural components, with high operation reliability and quick response behaviour.
It is advantageous to incorporate the stop device or each stop device structurally into the electromotor or the permanent motor, respectively. This allows the manufacturer to provide the necessary precise settings.
In case of a reversing permanent magnet motor or a rotating actuator expediently each stop device is located between the armature and a rotational bearing for an armature shaft and/or a shaft of the component, or between two rotational bearings of the armature shaft. In this area sufficient mounting space is available to receive the stop device.
In a permanent magnet motor the full cross-section of the armature preferably should be useable for the electromagnetic polarisation, in view of a quick response behaviour and a high power. At the same time, the armature shaft has to guarantee an accurate rotational support and long lift duration with optimum low rotational resistance. Both requirements are opposite to each other. In order to avoid a penetration of the armature by the armature shaft the component or a section of the armature shaft may comprise a hub part which either grips the front end of the armature like a pot or which is inserted like a plate into said front end. Since the armature shaft or component does not need to be an electromagnetically active part of the armature, it can be optimised in view of strength and functionality. To the contrary, the armature which is not penetrated by the armature shaft in this case can be designed for optimum magnetic function and can be polarised over its full cross-section.
Expediently the armature is supported for rotation at both ends by means of coaxial spindles. However, said spindles do not penetrate the armature. This facilitates manufacturing of the permanent magnet motor and improves its operation performance.
In case that the armature of an electromotor constitutes the drive element for the component of a yarn brake, quick linear or rotating adjustment movements can be generated.
In a yarn brake the driving element expediently is constituted by the magnetically polarised armature of a permanent magnet motor.
The stop device which is responsible for the positioning of the moveable parts of the yarn brake without backwards jumps expediently is incorporated into the actuator.